1. Field of the Invention
The present invention relates to a glass plate for use as a cap or a window glass in an alumina package for hermetically containing, as a contained element, a semiconductor element such as an erasable programable read only memory (EPROM), an opto-electronic element like an image sensor or other opto-electronic elements.
2. Description of the Prior Art
In the prior art, the contained element is within the alumina package to which a borosilicate glass plate or cap is mounted and hermetically sealed so that the contained element is hermetically enclosed in the package. The alumina package usually has a thickness of about several millimeters, while the glass plate has a thickness of 1 mm or more to provide it with a sufficient mechanical strength so as to protect the contained element in the hermetically sealed package.
Electronic devices are required to be small-sized in order to increase a mounting density on a circuit plate. Therefore, the glass plate is also required to have a reduced thickness of 0.7 mm or less.
Usually, the alumina package is subjected to an air tight reliability test or a helium pressure test where helium gas pressure of 5 Kg.f/cm.sup.2 is applied to the aluminum package hermetically sealed with the glass plate. Although the glass plate of 1 mm thickness or more is not broken in the test but the glass plate of 0.7 mm or less is broken in the test. Therefore, it is impossible to use the glass plate of 0.7 mm or less thickness as the glass cap for hermetically sealing the alumina package.
For example, Japanese Patent Publication No. 14185/1989 (Tokukou Hei (JP-B-) 1-14185, Reference I) discloses an alumina sealing ultraviolet transmitting glass which is used in alumina package for containing the EPROM, while Japanese Unexamined Patent Prepublication No. 65954/1987 (Tokukai Sho (JP-A-) 62-65954, Reference II) discloses an ultraviolet transmitting glass which is used in alumina package for containing the image sensor. When these glass plates are made to have a thickness of 0.7 mm, they are damaged during the helium pressure test although they are preferable in sealing with the alumina package.
As is known in the art, the glass plate is damaged or broken by increase of tensile stress. A chemical reinforcing method is already known as a manner for increasing strength of the glass plate. The method is called an ion exchange process in which Na.sup.+ ions are ion-exchanged in the glass plate by alkaline ions, such as K.sup.+ ions, having an ion radius which is larger than that of Na.sup.+ ions. For example, when glass plate including Na.sup.+ ions, is soaked within molten KNO.sub.3, exchange reaction occurs between Na.sup.+ ions of the glass plate and K.sup.+ ions of molten KNO.sub.3, so that K.sup.+ ions enters into a surface of the glass plate As a result, a compressive stress layer is formed in the surface of the glass plate and has compressive stress which is higher than that of the other part of the glass plate because the ion radius of K.sup.+ ions is larger than that of Na.sup.+ ions.
The compressive stress layer serves to relax the tensile stress applied to the glass plate, so that the mechanical strength of the glass plate is increased. The compressive stress of the compressive stress layer becomes relatively higher, the mechanical strength is higher.
Thus, it is known to use the above-described chemical reinforcing method to process the glass plate used for the cap of the alumina package. However, in the prior art, the glass to be chemically reinforced is not matched in thermal expansion coefficient with the alumina (approximately equal to 70.times.10.sup.-7 /.degree. C.) and is therefore not suitable for hermetically sealing the alumina package. For example, Japanese Unexamined Patent Prepublication No. 251541/1986 (Tokukai Sho (JP-A-) 61-251541, Reference III) and Japanese Unexamined Patent Prepublication No. 239036/1989 (Tokukai Hei (JP-A-) 1-239036, Reference IV) disclose the fact that the chemical reinforcement is used for reinforcing glass plates. However, the glass plates disclosed in the above-mentioned references III and IV are not suitable for hermetically sealing the alumina package because the glass plates have a high thermal expansion coefficient in comparison with alumina. Consequently the glass plates may be subjected to the tensile stress and therefore suffer from cracks due to the mismatch in the thermal expansion coefficient after being sealed to the alumina package.